Pulsed laser deposition has been used for depositing thin films of materials. Metal oxide films, for example, have been prepared using pulsed laser deposition [1].
A typical pulsed laser deposition apparatus includes a gas discharge laser, such as an excimer laser, that provides (1) a pulsed ultraviolet laser beam that has enough energy to ablate (i.e. vaporize material from) a target or targets, (2) optical components to focus the pulsed laser beam on the target(s), and (3) a vacuum chamber to house the target(s) and the substrate(s) where the deposition takes place. Highly crystalline films result from extended surface diffusion of adatoms at a given substrate temperature in between the pulses. Pulsed laser deposition can provide a deposited film with the same stoichiometry as that of the target if the laser energy density (i.e. fluence, expressed in J/cm2) is above a certain minimum threshold.
Growing films of complex materials or multiple component compounds by pulsed laser deposition requires the accurate control of chemical composition and layer thickness. Growing superlattices, for example, requires accurate control of the chemical composition and the individual layer thickness. For growing such films by pulsed laser deposition, it is desirable that the laser spot on the target have a uniform energy density distribution, a constant spot size, and a stable intensity because changes in the energy density, spot size, and/or intensity of the laser beam may not allow for control of layer thickness with unit cell accuracy.
There are many reports related to processing-structure-property relationships for different materials deposited by pulsed laser deposition [2-6]. However, detailed reports related to the controllability and reproducibility of the deposition step are scarce [7, 8]. The scarcity of such reports may be due to a false belief that the growth of thin films by pulsed laser deposition is straightforward. In practice, reproducible depositions can be extremely challenging due to a lack of control of the energy density of the laser beam. Gas discharge lasers such as excimer lasers ArF, KrF, and XeCl are commonly used for pulsed laser deposition. Controlling laser energy density of these gas discharge lasers is complicated because these lasers have notoriously poor beam quality. The laser energy density of such lasers varies spatially from the edge to the center of the beam spot. The beam dimensions also change with the discharge voltage applied across the electrodes of the discharge laser.
The gas in an excimer laser is excited by a high voltage discharge that generates photons for the lasing process. The beam size and shape are directly related to the freshness of the gas and to the drive voltage across the electrodes in the laser. It takes less drive voltage to achieve a desired energy for fresh laser gas than for old gas. Old gas must be driven to much higher voltages to achieve a similar energy as for fresh gas. As a consequence, only a small area of the electrodes will produce a discharge at a lower voltage, resulting in smaller beam dimensions. At higher voltage, a larger area of electrodes is involved, leading to an output with larger dimensions. These things should be considered when calibrating the laser energy density because the focused spot size is directly related to the size of the beam incident on the focusing lens. This is particularly tricky for commercially available excimer lasers (e.g. LAMBDA PHYSIK lasers) operating in a “constant energy” mode in which an operator simply enters a desired energy and the software that controls the laser determines the voltage required to achieve the output energy. The software associated with these lasers makes adjustments so that the voltage is driven higher to increase the beam fluence as the gas ages. As a consequence, the laser energy density of the focused spot on a target can fluctuate from time to time.
Stabilizing the energy density of the laser beam during pulsed laser deposition of a thin film is desirable.